W. Kaim et al., CORRELATION BETWEEN STRUCTURE AND ELECTRO N-TRANSFER MODALITY OF MODEL ORGANOMETAL COMPOUNDS FOR FLAVINS WITH BUTTERFLY CONFORMATION, Journal of organometallic chemistry, 456(2), 1993, pp. 167-173
The crystal structure analysis of a highly substituted 1,4-dihydropyra
zine is described by example of the 4-bis(tert-butyldimethylsilyl)-2,3
,5,6-tetramethyl derivative 2. In the solid state, the heterocycle exh
ibits pronounced deviation from planarity with a dihedral angle of 140
.8-degrees along the N-N' axis of the butterfly-shaped 8 pi-electron r
ing. The alkyl groups of the Si(tert-butyl)Me2 substituents are arrang
ed to minimize steric repulsion. Despite the strongly reduced pi-elect
ron conjugation caused by non-planarity, compound 2 is oxidized revers
ibly to an ESR detectable radical cation 2+. whereas the second oxidat
ion to the fully aromatic dication 2(2+) is electrochemically irrevers
ible. The strongly diminished difference between the anodic peak poten
tials for the two oxidation steps correlates with the non-planar confo
rmation as is shown by comparison with similar 1,4-bis(tri-alkyl- or d
ialkyl-silyl)-1,4-dihydro-1,4-diazines. The results support the concep
t of structural-controlled one electron vs. two electron transfer of f
lavin prosthetic groups in flavoenzymes.